To install brainrender, you need a python environment with a python > 2.7 and python < 3.8 (so python 3.6 or 3.7 ideally). If you don't have a python environment ready, you can crate one with anaconda.
Installing brainrender is as simple as:
pip install brainrender
Make sure to have your anaconda environment active when running pip install
You might get some error messages reporting conflicting version requirements for the allensdk package and a couple others. You can safely ignore these.
Note that some of brainrender's more rarely used features may require the installation of additional packages.